With the ever increasing awareness of pollution problems in contemporary society, and with the ever increasing volume of waste products which forms a natural byproduct of growth in the industrialized world, there have been ever increasing demands placed upon the requirements of new sewage treatment plants, and also demands for improvement of existing systems. In my patents U.S. Pat. No. 3,743,100, dated July 3, 1973, entitled FILTER PRESS, MORE PARTICULARLY FOR DEWATERING SLUDGE IN SEWAGE TREATMENT PLANTS, and U.S. Pat. No. 3,896,030, dated July 22, 1975, entitled identically to the first mentioned patent, I have described certain filter press systems to be advantageously used for the facilitation of sludge treatment. In the latter U.S. Pat. No. 3,896,030, I have also discussed a dewatering system which has been based on the principle of simply straining sludge through the action of gravity. It was pointed out in the U.S. Pat. No. 3,896,030 that various known filter presses possess a preliminary dewatering section, which is essentially a straining system, while in other cases the material on the filter belt is first squeezed between pressure rollers, prior to treatment on a rotary drum system. The filter press dewatering means described in this prior art in the U.S. Pat. No. 3,896,030 have the disadvantage that filtering efficiency is comparatively low, the construction relatively complex, and the space occupied by the machine is relatively great. In my U.S. Pat. No. 3,896,030 there is described a new dewatering means which has a pretreatment system, whereby the sludge, prior to reaching the rotary belt is pressed between two filter belts, with a filter belt acting as a strainer. Thus, through the action of gravity an appreciable amount of the water is removed, thus reducing the total water content of the sludge prior to entering the rotary drum system. In my copending patent application, Bahr, U.S. Ser. No. 632,789, now abandoned, entitled SLUDGE PRETREATMENT DEWATERING BASED UPON A CONTINUOUS SYSTEM OF FILTER POCKETS, filed on even date herewith, there is described an advantageous method for partially dewatering sludge prior to treatment of the sludge in a conventional filter press. This system in part serves to improve existing sludge treatment systems, and this system of said copending patent application may be incorporated into the method and apparatus of the present invention.
Sludges may also be dewatered through the use of tower presses, in which two filter belts are pressed vertically, one against the other, both belts being used in this filtering process. It should be noted that a totally satisfactory result is not attained through this alternative method to the filter press methods which have been described in the prior art, and referred to in said copending application.
With respect to the evolution of drum filter press techniques, although there have been efforts made in this area it has been found that when only one filter belt is used for all the phases of dewatering, that the mesh size of such belt must be designed for the highest pressure which is used in the process; otherwise, the filtrate which is the result of this process is heavily soiled. To be sure, the general condition of sludge alters as it goes through such a press in accordance with the quantity of the water which is removed from the sludge. Corresponding to the overall condition of the sludge, one would better choose at each point in time during the overall process optimum conditions of the filter size, the pressure and pressing time, and other parameters related to the general condition of the sludge. It can be seen that such control is desirable as the applicant has seen that with typical drum filter presses the water is first expelled but then is sucked back into the sludge when the pressure is cut off, leading to the conclusion that the practially feasible end product solids contents is not achieved. The failure to be able to regulate such parameters as time and pressure at the individual steps in the change of the sludge condition in the dewatering processes is thus detrimental to achieving an optimum result.